A typical wireless communication system comprises a plurality of wireless communications devices exchanging data with each other. In some wireless communication systems, for example, infrastructure networks, the system may further comprise a wireless base station for managing communications between the wireless communications devices. In other words, each intra-system communication would be exchanged via the wireless base station. In other wireless communication systems, for example, mesh networks and ad hoc wireless networks, the wireless base station may be omitted, i.e. the wireless communications devices may communicate directly with each other.
In the typical digital wireless communication system, the data to be transmitted, which at its most basic level comprises 1s and 0s, may be encoded into a modulation waveform. Depending on the data being transmitted, the transmitter device changes the transmitted signal based upon the modulation waveform.
A typical modulation waveform may include M-ary frequency-shift keying (M-FSK), which is a frequency modulation scheme transmitting digital information through discrete frequency changes of a carrier wave. A rudimentary example of the M-FSK modulation waveform is the binary FSK (BFSK or 2-FSK), which includes using a single pair of discrete frequencies to transmit digital data. Other modulation waveforms may include, for example, Gaussian minimum shift keying (GMSK), M-ary pulse amplitude modulation (M-PAM), M-ary phase shift keying (M-PSK), and M-ary quadrature amplitude modulation (M-QAM). As will be appreciated by those skilled in the art, the choice of the modulation waveform may depend on the performance demands of the system, for example, throughput and the type of data services being transmitted. For example, some modulation waveforms may be better suited for transmitting voice services rather than pure data services.
Typical wireless communication systems employ families of waveforms designed for worst case scenarios, i.e. maximum expected multipath and/or Doppler spread conditions. This design choice may allow for very robust waveforms that work under a wide variety of channel conditions. A potential drawback of this approach may be that the waveforms are typically over-designed for moderate channel conditions, i.e. higher throughputs could have been achieved over the wireless link if waveforms could have been adjusted real-time to match the real-time channel conditions. Furthermore, some wireless communications devices may temporarily cease communications to modify the waveform being used since user interaction may be needed, see, for example, U.S. Pat. Nos. 6,343,207 and 6,389,078 to Hessel et al., each assigned to the assignee of the present application.
Another approach is to design the family of waveforms based upon the mean transmission scenario, i.e. the expected average multipath and Doppler spread channel conditions, instead of the worst-case scenario. A potential drawback to this approach is that for transmission scenarios exceeding these average conditions, the transmissions would fail.
The US MIL-STD-188-110B (Appendix C) is an example of a family of waveforms designed to a set of expected worst-case channel conditions. These worst case channel conditions are about 6.7 milliseconds of multipath and 4 Hz of Doppler spread. These design choices may be achieved by using a cyclically extended 16 symbol Frank Heimiller sequence (FHS), which follows a 256 symbol data frame. The 16 symbol FHS may be used to compute an estimate of the multipath channel and the separation in time of the FHS (256 data symbols plus 31-symbol mini-probe created by cyclically extending 16 symbol FHS) for determining how fast the channel can change, i.e. Doppler spread.